In motor vehicles, incremental sensors are frequently used to acquire driving state variables. Such sensors only permit changes in the driving state variable to be detected. On the basis of a detected starting value it is possible to acquire the absolute value of the driving state variable by reference to the changes which are acquired. In order to determine the absolute value of the driving state variable, it is therefore necessary firstly to acquire at least one absolute value which is used to acquire the starting value. This absolute value can be calculated in a model-based fashion with reference, for example, to measured values of further driving state variables.
Incrementally measuring steering angle sensors are used particularly frequently in motor vehicles. In order to acquire an absolute value of the steering angle which is used to determine the starting value for the measurement of the steering angle, it is possible to acquire, for example by means of further sensor signals, whether the vehicle is traveling straight ahead, since in this situation it is possible to assume an absolute steering angle of zero. In addition, it is possible, for example, to calculate an absolute value of the steering angle, as explained further below, in a vehicle model and to use it to determine a starting value. In known applications, this starting value has to be newly acquired after each restart of the ignition. In order to be able to determine the absolute value with sufficient precision, said absolute value has to be evaluated over a relatively long period of time after the restart of the ignition. During this time period, an absolute value of the steering angle is not available. The steering angle can only be determined with respect to a starting value which is selected as desired and which corresponds, for example, to a value which was present when the preceding ignition sequence ended. However, this does not generally correspond to the absolute value of the steering angle which is present at a restart of the ignition since the steering angle can be changed while the ignition is switched off. The steering angle which is determined by means of such a starting value therefore generally has an offset error whose magnitude is not known.
Reliable model-based evaluation of the driving situation in order to determine the absolute value of the steering angle can only be performed when the driving state of the vehicle is stable, i.e. when the vehicle follows the steering specifications of the driver in a defined way. In the unstable driving situations, i.e. driving situations in which the vehicle does not follow the driving specifications in a “normal” way, there is no defined relationship between the measured driving state variables and the driver's steering specifications. For this reason, such an evaluation of the driving situation for determining an absolute value of the steering angle can be performed only when there is a stable driving state. As a result, it is necessary to detect unstable driving states.
Known methods for reliably detecting unstable driving situations are based on comparing measured values of a driving state variable with values which are calculated by means of driver specifications in vehicle models. However, such a comparison generally requires knowledge of the absolute values of all the driving state variables. For this reason, these methods cannot be applied unless an absolute value has been determined for the steering angle.
These and other aspects of the invention are illustrated in detail by way of the embodiments and are described with respect to the embodiments in the following, making reference to the Figures.